To construct a visible image on the screen of a monitor, television receiver, or other image-displaying device, one or more lasers can be employed to scan a controlled beam of light across a diffuser screen on which the visible image is formed. However, lasers, which generate a coherent light beam, create a “speckling” effect in the visible image, which detracts from image quality. Non-laser sources can also produce objectionable speckle, e.g., when a projector with a small lens projects an image onto a large screen with a small throw distance (e.g., as is often required in a rear-projection television receiver with a shallow cabinet depth). In that case, the projection lens appears as a quasi-point-source (like a laser) and can produce noticeable speckle.
Continuously moving the diffuser screen is a known despeckling technique. To produce a uniform level of despeckling across the screen, it is important to maintain a relatively constant level of motion for the entire surface of the screen so that the velocity of any point on the visible screen surface never falls below a critical value. For example, a straight-line oscillatory motion of the screen can produce points of zero velocity twice each oscillatory cycle. A preferred screen motion is a two-dimensional orbital motion or a “figure eight”/Lissajous-type motion, neither of which produces moments during which the velocity of the screen is zero. However, external noise and vibration, environmental effects (e.g., effects due to temperature, gravity, etc.), nonlinear springs, multiple frequencies/resonant modes, etc., can lead to unpredictable motion, generating a need to sense screen motion for feedback control. Prior art techniques to sense screen motion include optical screen motion sensors, among other motion-sensing devices.
Turning now to FIG. 1, illustrated is a simplified drawing of a four-quadrant optical position sensor 101 of the prior art. The optical position sensor is formed with four photodiodes arranged in quadrants 105, 106, 107, and 108. When the photodiodes are illuminated by a light beam, such as light beam 102 produced by a light-emitting diode (not shown), each quadrant generates a current proportional to the net light flux of the beam falling thereon, which can be sensed and compared using ordinary operational amplifiers to measure sensor position. Typically, the beam is formed with a bell-shaped intensity distribution, with the highest light intensity at the center of the beam. In this manner, misalignment of the beam with respect to alignment of the photodiodes can be detected using conventional circuits such as operational amplifiers configured as difference amplifiers coupled to the photodiodes to sense photodiode current. Velocity of the screen can be determined by differentiating with respect to time the position of the screen produced by the photodiode sensing arrangement. Such optical-sensing approaches, however, generally lead to alignment problems as parts bow and distort over time due to changes in temperature, or changes in direction of gravitational forces due to cabinet reorientation. Misalignment of sensors can also result in mechanical abrasion due to unintended screen motion, leading to wear, failure, and noise. The use of photodiodes to sense screen motion also adds significant cost to an end product due to the need to provide, mount, and align additional mechanical components.
A further example of motion sensing is the use of accelerometers. A disadvantage, however, of using accelerometers is their relatively high cost. Accelerometers also have a disadvantage in that inferring velocity and position requires acceleration to be temporally integrated once for velocity, and then again for position. Thus, velocity and position are prone to accuracy and stability problems such as “integration of error.”
Thus, what is needed in the art is a motion sensing arrangement that overcomes these effects.